1. Field of the Invention
The present invention relates generally to the construction of catheters and more particularly to the construction of vascular catheters having internal drive cables for rotating a work element at the distal end of the catheter.
A variety of vascular catheters employ rotatable work elements at their distal ends. For example, vascular imaging catheters may employ a rotating ultrasonic transducer, or fixed transducer in combination with a rotating mirror, in order to scan an imaging beam around the distal end of the catheter. Alternatively, interventional vascular catheters may employ rotating blades, abrasive elements, laser directing elements, and the like. Such catheters frequently utilize a flexible drive cable in order to transmit a rotational drive force from a motor located at the proximal end of the catheter to the work element located in the distal end. In addition to rotation, it is frequently desirable to be able to axially translate the work element within the catheter, preferably by simply pushing or pulling on the proximal end of the drive cable.
The need to transmit rotational motion, i.e., torque, and optionally axial motion places substantial mechanical requirements on the drive cable. The drive cable must possess a certain torsional stiffness in order to adequately deliver rotational force along the relatively long path traversed by the cable. In particular, it is undesirable for the drive cable to experience substantial "wind up" which occurs as a result of twisting along its length. Additionally, it is undesirable for the cable to experience excessive bending which can cause seizing of the cable within the catheter lumen in which it is located. Both of these problems are exacerbated by the tortuous path which the catheter must follow in vascular applications. In particular, the drive cable must be sufficiently flexible to bend around the tight turns presented by the vascular system while maintaining the ability to rotate, frequently at very high rpm, as well as the ability to axially translate through the catheter lumen at all times.
Heretofore, drive cables have usually consisted of solid core wires or braided cables. Such cable constructions, however, have generally represented a compromise between providing the torsional stiffness needed to avoid excessive wind up and flexural stiffness needed to adequately transmit axial force and the flexibility necessary to avoid seizing and binding of the cable within the catheter lumen.
It would therefore be desirable to provide improved drive cables for catheters having cable-driven work elements at their distal end. In particular, it would be desirable to provide drive cables which have sufficient torsional stiffness to avoid excessive wind up, sufficient flexural stiffness to avoid seizing and binding of the cable within the catheter lumen, while remaining sufficiently flexible in order to negotiate the tortuous path which is dictated by passage of the catheter through the vascular system. It would be further desirable to provide drive cables having very narrow diameters, particularly at their distal end, in order to allow construction of small catheters which can enter very small diameter blood vessels. It would be further desirable to provide drive cables which are capable of operating at high rpm while bent across the narrow curvature of the vascular system without experiencing excessive metal fatigue.
2. Description of the Background Art
U.S. Pat. No. 4,794,931, the disclosure of which is incorporated herein by reference, discloses a vascular catheter having a flexible drive cable extending through a central lumen. The cable is disclosed as being formed from stainless steel, with a helically braided wire configuration. Work elements, including ultrasonic transducers, mirrors, and cutting blades, are attached to the forward end of the drive cable. Guide wires having variable flexibility are described in U.S. Pat. Nos. 4,724,846; 4,682,607; 3,416,531; and 4,215,703. U.S. Pat. No. 4,747,406, describes an articulated support tube which facilitates bending a drive cable around tight radii. Nested coil torque cables are commercially available from S.S. White Industrial Products, Inc., Piscataway, N.J., and described in brochure no. 750-66395.